"We gain strength, and courage, and confidence by each experience in which we really stop to look fear in the face... we must do that which we think we cannot."
Eleanor Roosevelt

Tuesday, March 04, 2008

Crosswind landings

Wind strength and direction is so important to a pilot. Tailwinds (those coming from behind us) are usually good - they push us to where we are trying to go, getting us there faster. However, tailwinds are NOT good on takeoffs or landings.

According to AOPA (Airplane Owner and Pilots Association):

Don't underestimate the effect of tailwinds--no matter how small--on either takeoff or landing. Not only do they cause an airplane to use more runway in both situations, but on takeoff they'll also flatten the climb angle, which may hamper your aircraft's ability to clear obstacles. In addition, a quartering tailwind makes a tailwheel airplane difficult to control. If landing runway length is even remotely a factor, as little as two knots on the tail can take away your margin, so landing on, or near, the numbers (at the beginning of the runway) becomes critical. The best remedy is to avoid takeoffs or landings in tailwinds of any kind.

Headwinds are not desirable during the cruise portion of the flight, but they work to pilots' advantage on takeoffs and landings, increasing airplane performance and decreasing the amount of runway used. I would say "needed" except most pilots do not PLAN for the headwind (landing on a shorter runway), but just take the headwind's benefit as a bonus during landing. One wouldn't want to plan on a headwind and then have the winds suddenly die down or shift directions.

Speaking of shifting directions, what if the wind is not straight down the runway? It is then called a crosswind. Most airplanes have a maximum crosswind limitation (often 20-30 knots). An airplane compensating for a crosswind has to "crab" into the wind so it can continue to travel in the intended direction. To imagine what I'm talking about say you have to swim across a river. If you pick a point directly across from you, you will end up farther down the bank than you had intended because of the current pushing you UNLESS you swim an appropriate amount upstream to compensate for the speed of the river.

On takeoffs and landings, crosswinds can be a little tricky. Beginning pilots (and experienced pilots) practice and have to pay a lot of attention to the effect of the crosswind.

AOPA again:

The most commonly taught crosswind landing technique is the cross-control, or wing-low landing. The pilot slips the airplane to the runway with just enough cross control to keep the aircraft aligned with the centerline. Remember thatthe ailerons control the airplane's lateral movement. Use them to counteract the downwind drift caused by the crosswind and put the airplane on the runway centerline. Use the rudder to align the airplane's longitudinal axis with the runway centerline -- keep the nose pointed straight down the runway. (The pilot transitions from a "crab" to a "slip" right above the runway so the airplane will land straight down the runway - to not land on the gear while going sideways.)

Remember that all control forces will change during the transition from final approach to the end of the roll out. Wind direction and speed often change with altitude, and the control deflections required to maneuver the aircraft will increase as the aircraft's speed decreases. In general, you'll need to increase the aileron and rudder deflection as the aircraft speed decreases. Don't release your control inputs once the wheels are on the runway. The wind still affects the airplane, and you need to use the appropriate control inputs all the way to the tie down.

Would you like an illustration? Here is a video of a large Lufthansa jet landing with a major crosswind. Watch how much the airplane is "crabbing." It almost looks like it should be flying to your left, but it is coming straight towards you. Then at the last minute the pilot tries to slip - but the wind is so strong that it exceeds the maximum crosswind component for the airplane. The wingtip even touches the ground! http://news.yahoo.com/s/ap/20080303/ap_on_re_eu/germany_rough_landing